The present invention relates to the art of generating magnetic fields, particularly strong uniform magnetic fields. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application with magnetic resonance spectroscopy, and other medical and diagnostic techniques, and the like.
As the use of magnetic resonance imaging ("MRI") techniques expands, there is an increased interest in bringing equipment (including equipment for other imaging modalities, interventional surgery, heart/lung machines, emergency oxygen tanks, etc.) into a room with an MRI imager. However, the equipment is frequently not compatible with the magnetic field produced by the MRI imager. Under those circumstances, discharging the magnet in the MRI imager may be appropriate before the equipment is introduced into the room. Then, when additional images are desired, the MRI magnet would be ramped-up after the interfering equipment is removed from the room. Therefore, it is desirable to charge and discharge the MRI magnet quickly.
Heretofore, smaller magnets, e.g., 0.5 T with a minimum inductance design have been used for imaging during interventional surgery. Both resistive and cryogenic magnets require a stabilization time after being brought back up to field, i.e. ramped-up. In cryogenic magnets, the superconducting wire commonly includes a copper matrix with embedded, twisted strands of type II superconductor. During energization or de-energization, current is transferred between the copper and the superconducting strands. Current in the copper has a dissipative decay with a time constant of seconds. The current is distributed and balanced among the twisted strands of the superconductor. The stabilization time for such a superconducting magnet has a time constant on the order of minutes. The effect on imaging during this time is primarily a B.sub.0 field shift. This time period of instability is undesirable because it results in delays.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.